Manipulation of Electrical Flicker-Noise and Line Narrowing in Free-Running Quantum-Cascade Lasers

Intrinsic linewidths of quantum-cascade lasers are found to be extremely narrow, similar to 100 Hz. However, the free running linewidths (usually similar to 1 MHz) of existing quantum-cascade lasers are governed by flicker frequency-noise that is identified to originate from electrical flicker-noise in the devices. Obviously, substantial suppression of the electrical flicker noise is required for substantial narrowing of free-running LWs. In this presentation, we show systematic experimental results of flicker voltage-noise power-spectral density obtained with mid-infrared quantum-cascade lasers of designed positioning of impurities in injectors. The measured noise-levels depending strongly on impurity position as well as device-temperature are evaluated with an ad hoc model based on fluctuating charge-dipoles induced by trapping and de-trapping at impurity states in their injectors. It is shown that quasi-delta doping of impurities leads to strong suppression of electrical flicker noise by minimization of the dipole-length at a certain temperature, for instance similar to 300 K and, in turn, is expected to narrow astonishingly the free-running line-width down below 10 kHz without assistances of any types of feedback schemes.